1. Field of the Invention
The present invention relates to a parallel operation method for an uninterruptible power supply apparatus.
2. Description of the Related Art
FIG. 6 is a block diagram showing the same structure as the fundamental structure of an uninterruptible power supply apparatus described in “Journal of the Institute of Electrical Engineers of Japan, Publication of Industry Applications Society (D), vol. 107, No. 11, p. 1312”. An uninterruptible power supply apparatus 1 shown in FIG. 6 is constructed such that power is fed in a bypass feeding through a circuit breaker 101 and power is also fed from an input power supply through a circuit breaker 102. The apparatus 1 includes an AC switch 17 connected in the bypass feeding through the circuit breaker 101; a switch 11; a converter 12 which is connected to the input power supply through the circuit breaker 102 and the switch 11 to convert AC power into DC power; a battery 13; a switch 14; an inverter 15 for converting DC power into AC power; and a switch 16.
Next, operation thereof will be described. When the input power supply is normally operated, the converter 12 converts AC power into DC power through the circuit breaker 102 and the switch 11, and supplies the DC power to the inverter 15, while charging the battery 13 through the switch 14. The inverter 15 outputs AC power through the switch 16. On the other hand, when the input power supply is interrupted, the converter 12 is stopped, and the inverter 15 outputs AC power using power in the battery 13.
When the inverter 15 is stopped in a normal manner, stopped by an overload, or stopped by a failure, the AC switch 17 is turned on so that power is fed in the bypass feeding to a load. FIG. 7 shows a control circuit for the AC switch 17, which is composed of a sequence control circuit 1003 and a switch drive circuit 1040 and incorporated in the uninterruptible power supply apparatus 1 having the structure shown in FIG. 6. The operation of the AC switch 17 will be described with reference to FIG. 7. As shown in FIG. 7, the AC switch 17 is constructed to be turned on when an inverter feeding stop signal received from the sequence control circuit 1003 is received by the switch drive circuit 1040 to stop inverter feeding. Thus, even when the inverter 15 has failed, power can be continuously fed to the load.
In order to improve power feeding reliability of such an uninterruptible power supply apparatus or to increase output capacity thereof, parallel operation is conducted and the structure as shown in FIG. 8 is used in many cases. A parallel operation apparatus for the uninterruptible power supply apparatus as shown in FIG. 8 includes parallel uninterruptible power supply apparatuses 4 and 5. For example, when a load is within 100 kVA, rated capacity of the uninterruptible power supply apparatuses 4 and 5, which are operated in parallel, is set to 100 kVA. Thus, when at least one apparatus is operated, it becomes a redundant structure capable of supplying power to the load without interruption, which is called parallel redundancy. A difference between the uninterruptible power supply apparatus 1 shown in FIG. 6 and the parallel uninterruptible power supply apparatuses 4 and 5 shown in FIG. 8 is as follows. That is, although the uninterruptible power supply apparatus 1 includes the bypass feeding and the AC switch 17 for supplying power in the bypass feeding to the load when power feeding to the inverter is stopped, the parallel uninterruptible power supply apparatuses 4 and 5 do not include the bypass input and the AC switch.
According to the structure shown in FIG. 8, a switching panel 6, to which the outputs of the parallel uninterruptible power supply apparatuses 4 and 5 and the power from the bypass feeding are input, is provided. Power is fed in the bypass feeding to the load when power cannot be fed from any of the parallel uninterruptible power supply apparatuses 4 and 5. In the switching panel 6, the outputs of the parallel uninterruptible power supply apparatuses 4 and 5 are connected to each other to produce a parallel bus. Generally, the outputs of the parallel uninterruptible power supply apparatuses 4 and 5 are supplied to the load through a switch 31. When the power cannot be fed from any of the parallel uninterruptible power supply apparatuses 4 and 5, the switch 31 is turned off and power is fed from the bypass feeding to the load through semiconductor switches 33 and 34 and a switch 32.
Also, when the load is within 100 kVA at an initial installation of the uninterruptible power supply apparatus, the uninterruptible power supply apparatus 4, of the parallel uninterruptible power supply apparatuses 4 and 5, and the switching panel 6 are connected. When an additional load is provided and exceeds 100 kVA, the uninterruptible power supply apparatus 5 is added so that power feeding can be conducted for a load capacity of up to 200 kVA. Thus, when the output capacity is increased by the parallel operation, it is not a redundant structure and called parallel non-redundancy.
In the case of the parallel non-redundancy, according to the structure shown in FIG. 8, since the switching panel 6 is provided, to which the outputs of the parallel uninterruptible power supply apparatuses 4 and 5 and the power from the bypass feeding are input, when power cannot be fed from either of the parallel uninterruptible power supply apparatuses 4 and 5, the parallel uninterruptible power supply apparatus is brought into an overload state. Thus, since both apparatuses are finally in a stopped state, power is fed from the bypass feeding to the load. In the switching panel 6, the outputs of the parallel uninterruptible power supply apparatuses 4 and 5 are connected to each other to produce the parallel bus. Generally, the outputs of the parallel uninterruptible power supply apparatuses are supplied to the load through the switch 31. When the power cannot be fed from any of the parallel uninterruptible power supply apparatuses 4 and 5, the switch 31 is turned off and power is fed in the bypass feeding to the load through the semiconductor switches 33 and 34 and the switch 32.
In the conventional parallel operation apparatus for an uninterruptible power supply apparatus, the power feeding reliability can be improved or the output capacity can be increased. However, when such an uninterruptible power supply apparatus having no bypass circuit and the switching panel is used, the structure of the uninterruptible power supply apparatus is different from that of an uninterruptible power supply apparatus used as a single system and has a bypass circuit incorporated therein.
In contrast to this, FIG. 9 shows a structure in which the uninterruptible power supply apparatuses, each being used as a single system and having a bypass circuit incorporated therein, are operated in parallel. Respective uninterruptible power supply apparatuses 1 and 2 can feed power using an inverter and a bypass power source as a single apparatus. When a load is small at the initial location of the uninterruptible power supply apparatus and the capacity is sufficient with one uninterruptible power supply apparatus, only the uninterruptible power supply apparatus 1 is provided. Then, when an additional load is provided and the capacity corresponding to two uninterruptible power supply apparatuses is required, the uninterruptible power supply apparatus 2 is added thereto. Thus, an initial investment cost can be suppressed.
According to the structure shown in FIG. 9, the respective uninterruptible power supply apparatuses 1 and 2 include the same structure as shown in FIG. 6. Although reference numerals in the tens are used for the uninterruptible power supply apparatus 1, the structure of the uninterruptible power supply apparatus 2 is denoted by using corresponding reference numerals in the twenties. In FIG. 9, when an operational mode of the uninterruptible power supply apparatuses 1 and 2 is changed from bypass feeding to inverter feeding, if the respective uninterruptible power supply apparatuses are separately changed from the bypass feeding to the inverter feeding, one of the apparatuses provides inverter feeding and at least one different apparatus provides the bypass feeding.
In particular, in the case where an operation using a switch and the like for the inverter feeding is conducted by an operator for only the uninterruptible power supply apparatus 1, and the uninterruptible power supply apparatus 2 remains in a bypass feeding state without operation, an inverter feeding state continues in the uninterruptible power supply apparatus 1 and the bypass feeding state continues in the uninterruptible power supply apparatus 2. Thus, an inverter output of the uninterruptible power supply apparatus 1 and the bypass power supply output of the uninterruptible power supply apparatus 2 overlap. At this time, when the bypass power supply output of the uninterruptible power supply apparatus 2 is different from the inverter output voltage of the uninterruptible power supply apparatus 1, an overcurrent is caused in the inverter of the uninterruptible power supply apparatus 1 because of the different voltage outputs.
Thus, when the operational mode of the uninterruptible power supply apparatuses 1 and 2 is changed from bypass feeding to inverter feeding, AC switches 17 and 27 are simultaneously turned off in the uninterruptible power supply apparatuses 1 and 2, and it is necessary to simultaneously supply power from the inverters 15 and 25. Therefore, in general, there is a problem that a separate circuit for producing a common instruction is required.